Metallurgical furnace gas and method of controlling composition



Jan. 22, 1935. F. P. WILSON, JR 1,988,929

METALLURGICAL FURNACE GAS AND METHOD OF CONTROLLING COMPOSITION FiledMarch 6, 1930 Inventor: Frederick PWilSoTmJP,

by MW H i s After-neg.

Patented Jan. 22, 1935 umrao STATES METALLURGICAL FURNACE GAS ANDDIETHOD OF CONTROLLING COMPOSI- TION Frederick P. Wilson, Jr.,Schenectady, N. Y.,

assignor to General Electric Company, a corporation of New YorkApplication March 6, 1930, Serial No. 433,726

2 Claims.

The present invention relates to metallurgical operations, such, forexample, as the annealing and brazing of metals, and in particular tothe enveloping of steel and other carbon-containing metals duringmetallurgical operations with a gaseous atmosphere which is adapted toprotect the metals from oxidation and to preserve or modify the carboncontent of the metal.

The present invention comprises a method of controlling thecarbon-reactive property of a gaseous mixture so as to make the samecarbu rizing, neutral or decarburizing at will.

In accordance with my invention a gaseous mixture comprising adecarburizing gas, such as hydrogen, or carbon monoxide or both and acarburizing gas, such as methane, ethane, propane or butane/is subjectedprior to its introduction into a metallurgical furnace or the like to atemperature chosen and controlled to cause said gas to have a desiredcarbon-reactive eifect, that is, to be carburizing, decarburizing to adesired degree or neutral, as may be desired. The carbon content ofsteel, or other ferrous metal, may be preserved during a brazingoperation in an enveloping atmosphere which otherwise would lower thecarbon content in accordance with my invention by the presence of arelatively small amount of hydrocarbon gas in the brazing atmosphere,for example, about one per cent by volume of methane. I prefer also tohave present some carbon monoxide in the brazing gas as the presence ofwater vapor has less deleterious effect on it than on hydrogen.

The novel features of my invention will be pointed out with greaterparticularity in the appended claims.

The appended drawing shows in Fig. 1, in, somewhat diagrammatic form,and partly in section, an apparatus suitable for carrying out myinvention: Fig. 2 is a longitudinal section of an adjunct suitable foruse together with the apparatus shown in Fig. 1; and Fig. 3 somewhatconventionally illustrates a modification.

Referring to Fig. 1 of the drawing, a carbon-, containing gas, such as afuel gas of the character commonly employed for city gas supply, isintroduced by a supply pipe 1 to a mixing chamber 2. As examples of ahydrocarbon fuel gas, coal gas, natural gas, coke oven gas and producergas may be mentioned. Such a fuel gas usually comprises hydrogen, carbonmon-.

oxide, methane and higher normally gaseous hydrocarbons, commonly knownas illuminan A hydrocarbon, such as butane, also may be used. Incarrying out my invention, steam may be admixed with the hydrocarbon gasto combine with dissociated carbon although the presence of steam is nota necessary feature of my invention. The steam is admitted to the mixingdevice May a supply pipe 3, the steam flowing through a valve 4, whichis controlled by a temperature-responsive device 5' whereby the amountof steam admixed with the hydrocarbon gas is controlled. The proportionof steam to be added to the gaseous mixture may be determined by trialand analysis. Ordinarily,

steam should be added in such stoichiometrical proportion that carbonfreed in the reaction chamber is reacted upon substantially completelyto form hydrogen and carbon monoxide gas.

The device 5 operates the valve 4 to vary the steam supply in responseto 'the temperature of the gas and steam mixture in the mixing chamber 2so as to maintain a constant ratio of combustible gas and steam in thereaction mixture delivered to a reaction chamber 6 by the pipe 7,emerging from the mixing chamber. I may use for elements 4 and 5 aregulator sold by the Taylor Instrument Company of Rochester, N. Y.under the trade designation Tycos Temperature Regulator. This devicecomprises a steam valve operated by a mechanism which is responsive tothe temperature of a medium (in this case the gas-steam mixture in 2)through the intermediary of a gas or other fluid under pressure, whichcommunicates with a bellows device (not shown) whereby motion istransmitted to the steam valve to vary its setting. With a propersetting of the regulating device, the quantitative ratio of 'gas andsteam is maintained constant regardless of variations of gas or steampressure.

steam to be admitted. Conversely a rise insteam ratio will result in anincreased liberation of heat in the mixing chamber and cause the device5 to reduce the flow of steam by reducing the aperture of the steamvalve. Other known suitable, regulating devices responsive totemperature of a medium may be employed, as for example, an electricallycontrolled device having a thermostatic element.

The gas mixture passes from the pipe through a heat interchanger 7a tothe reaction chamber 6 where it is subjected to a high temperaturevarying with the conditions and the results desired but in general beingwithin the-limits of about 700 to 1100 C.

As will'be hereinafter more fully explained the temperature in thereaction chamber is maintained at such value with respect to thetemperature and other conditions of the treating zone that the contentof hydrocarbon gas in the product will cause it to have a desiredchemical effect with respect to the metal charge in the furnace, thatis, to be decarburizing, carburiz ing or neutral as may be desired.

The hydrocarbon gas which in the examples given is in excess-of thecombining proportion required to eliminate such water vapor as may bepresent will be decomposed in the reaction zone, liberating carbon andcausing an equilibrium to be established between the gas and the carbonat the reaction temperature. If water vapor is present the carbon willcombine with the water vapor. If water vapor is absent, the carbon willbe deposited as lampblack in the reaction chamber.- In that event thereaction chamber should be periodically cleaned out.

The reaction chamber comprises a metal shell 8 within which is located afire-brick wall 9, resting on a base 10. Mounted on the wall 9 is adistributing chamber 11 which communicates by a conduit 12 with the heatinterchanger 7a. The shell 8 is lined with a heat insulator 13consisting of finely divided material, such as silocel. An electricresistance heater 14 is supported within the chamber 6 from the wall.

This resistor preferably comprises a ribbon of nickel-chromium alloywhich incidentallyhas a catalyzing effect upon the gaseous reactionoccurring in the reaction chamber. Its main function is to maintain adesired temperature in the reaction chamber. Suitable externalregulating devices (not shown) may be employed to control thetemperature of the heater.

The gas mixture entering the chamber passes through the iron tubes 15to' a region near the bottom of the chamber andpasses upward and emergesfrom the reaction chamber by the conduit 16 to an electric furnace, oroven 17 in which brazing, heat treating, or other metallurgical orchemical process is being carried out requiring a gas atmosphere.Examples of brazing furnaces are shown in United States Patents1,536,944, issued May 5, 1925 to Christian Steenstrup, and 1,610,809,issued December 14, 1926 :to D. F. Newman.

I The gaseous product delivered to the furnace 17 is constituted of apreponderant proportion of hydrogen, a lesser proportion of carbonmonoxide together with a minor content of inactive gas, such as nitrogenand carbon dioxide. A

small content of methane, say about one per eifectiveness of thehydrogen to prevent oxida-' tion disappears. This is not true of carbonmonoxide. If suflicient carbon monoxide is present to be in chemicalequilibrium with carbon dioxide, oxidation will not take place as thetemperature is lowered.

As the brazing gas produced by the above described process comprisesmainly hydrogen and carbon monoxide and a lesser amount of carbondioxide, oxidation of the charge will not occur during cooling. Ifreduction of oxide occurs in the brazing or annealing furnace, or in anyway oxygen is-introduced both water vapo'r and carbon dioxide firstareformed, but water vapor then reacts with the methane forming hydrogenand carbon monoxide.

At a given temperature prevailing in the reaction chamber, the hydrogenand me methane are in chemical equilibrium. Such mixture, is in chemicalequilibrium with respect to the carbon of the steel in a brazing furnaceat a temperature not far removed from the reaction temperature dependingon the amount of carbon in the steel and other conditions, including thepresence of water vapor in the brazing furnace.

If it is desired to carry out a metallurgical operation such asannealing of steel with an atmosphere which is neutral, that is neithercarburizing nor decarburizing with respect to the steel, then by trialand physical or chemical determination of the carbon content of thesteel before and after treatment, the conditions of the gas converterare so adjusted that a gas is produced which is in chemical equilibriumwith the steel or other charge.

By trial the various factors, such as temperature, rate of flow, watervapor content, composition of the hydrocarbon gas fed into the convertermay be adjusted to produce a neutral gas.

Ordinarily, it is preferable to carry out metallurg-ical operationinvolving ferrous metals by the use of a protective atmosphere which isslightly decarburizing. To accomplish this one of two courses may bepursued.

(1) The reaction chamber temperature may be increased to completelydecompose or crack the methane, or other hydrocarbon gas.

(2) The gas mixture may be varied to produce a small percentage ofcarbon dioxide in the gas fed to the brazing or other metallurgicalfurnace. For example, the steamcontent of the mixture fed into thereaction chamber may be increased.

If it is desired to operate a brazing furnace at say 1100 0., containinga slight amount of of this water vapor by the presence of a some.-

what larger amount of hydrocarbon in the gas. The reaction chambertemperature may be somewhat lowered by trial until the desired contentof methane in the gas is obtained. For many brazing operations about oneper cent of methane is desirable.

If a carburizing effect is desired, as when the gaseous products fromthe converter are to be used in a case-hardening furnace, then theconditions are chosen to produce a substantial ex-.- cess of methane inthe gaseous product. Ordinarily this will be done by operating thereaction chamber at,a still lower temperature, in some cases as low as700 0., at "which temperature a substantial proportion, say about 15%,

' portion of nitrogen varying from about 5% to 95%. The presence of thenitrogen renders an accidental explosion in the furnace less violent.

in fact, a mixture containing inert gas, such as nitrogen, in proportionas high as 95%, is noninflammable while still having a reducing effectupon metal if the mixture is dry.

If it is desired to produce a gaseous mixture for metallurgicaloperations having a substantial content of inactive gas, such asnitrogen, and carbon monoxide some of the fuel gas fed to the converteris first burned by mixing it with such amount of air that incompletecombustion is produced. In that event a modified gas feeding system isemployed as is illustrated in Fig. 3.

this figure a combustion chamber 29 is inserted into the gas supply line1 by closing the valve 21 in the feed pipe 1, and opening the valve 22in a supply pipe 23 of 'the combustion chamber, which is also suppliedwith oxygen, air or other combustion supporting gas under pressurethrough a pipe 24. As shown in Fig. 2 an oxygen flame is produced at theorifice of the air supply pipe 24 which burns in the surroundingatmosphere of fuel gas in the combustion chamber 20 and consumes part ofthe fuel gas forming carbon dioxida'carbon monoxide, and water vapor.These products, together with the nitrogen introduced as part of theair, are carried through the pipe 25 into the mixing chamber 2, and fromthence into the reaction chamber 6. It is desirable to remove the watervapor from the gaseous mixture in pipe 25. This can be done in anyconvenient way, as for example by passing the gases through a condenser26 and drawing ofi the condensed water. In some cases the water vapormay be retained in the gaseous mixture and a correspondingly lesserquantity of steam admitted to the mixing chamber, so as not to disturbthe gas and steam ratio. The gases entering the reaction chamber fromthe pipe 25 'will comprise hydrogen, carbon monoxide, hydrocarbon gas,such as methane, nitrogen, and carbon dioxide. This gaseous mixture isfurther admixed with steam depending on the amount of hydrocarbonpresent and the reaction then proceeds as above described, the productcontaining substantial admixtures of nitrogen depending on the amount ofair introduced in the combustion chamber.

When the metallurgical furnace is first started up it is desirable tosweep out air from the interior of the furnace by operating thecombustion chamber 20 with such proportions of combustible gas and airthat the products consist wholly of inert gas, such as nitrogen andcarbon dioxide. It is also desirable during the shutting down periodafter the metallurgical operation in the furnace 1'7 has been completed,or in fact at any time when it is desired to shut down the furnace 17,to purge the interior of'the furnace of' combustible gases so as toavoid an explosion. This may be done at any time by closing the valve 21supplying fuel gas, opening the valve 22 and supplying suflicient airthrough the supply pipe 24 to cause complete combustion as abovedescribed.

It is possible to cause a combustion chamber 20 to assume also thefunction of the reaction chamber 6, the operation being carried out insuch a way that cracking or decomposition of unburned hydrocarbon gasoccurs in the com bustion chamber. The product from the combustionchamber then comprises carbon dioxide, nitrogen, and carbon monoxide,and if desired, a small amount of hydrocarbon, this gaseous mixture thenbeing supplied to the metallur-r gical furnace without further change orreaction. When this result is desired enriched air or pure oxygen issupplied to the combustion chamber by the pipe 24 and the combustionchamber is constructed of refractory heat insulating ma= terial so as toconserve the heat evolved thereinv by combustion. With the combustionchamber operating at a high temperature, decomposition of either a largeproportion or all of the hydrocarbon gas occurs. In that case the gasesneed not be passed through the reaction chamber, which may be omitted.For convenience, if such operation is intermittent, the reaction chambermay be kept in the system but the resistance unit therein is not broughtup to operating temperature.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,-

1. The method of protecting ferrous metal while being brazed at atemperature of about 1100 C. which consists in enveloping said metalwith gaseous products obtained by heating a mixture of a hydrocarbon gasand water vapor to a temperature of about 1050 C. and removing watervapor from the gases produced by such heating before such gases come incontact with said ferrous metal.

2. The step in a metallurgical operation such as carburizing or brazing,involving heating solid ferrous metal to an elevated temperature atwhich the carbon content of said metal is subject tochange whichconsists in enveloping said metal during such operation with a gasobtained by heating a mixture of an oxidizing gas and a chemical excessof a hydrocarbon gas to a reaction temperature within the range of about700 to 1050 C. the reaction temperature being maintained at least 50 C.lower than the temperature at which said metallurgical operation iscarried out, thereby maintaining a predetermined quantity of hydrocarbongas in said enveloping gas.

' FREDERICK P. WILSON, JR.

